As an example of a conventional video image reproducing apparatus, a high speed reproducing operation of a video tape recorder employing a field memory (hereinafter referred to as a VTR) will be described with reference to even multiplied speed reproduction such as reproducing at four times the recorded speed. An odd multiplied speed is generally selected in the high speed reproducing. This is because the odd multiplied speed reproducing has such a characteristic that a noise bar is locked since a noise position is identical in each field. On the other hand, a width of a noise bar can be made narrow, in some cases, by employing a memory. This is because the even multiplied speed reproducing has such a characteristic that a noise position and a signal position occur alternately.
FIG. 1 is a block diagram showing the high speed reproducing system of the above described conventional VTR. In FIG. 1, reference numeral (1) is a recorded video tape. A reproducing signal is fed to a preamplifier (3) through video heads (2a) and (2b). Thereafter, the reproducing signal is applied to a video signal processing circuit (4). Reference numeral (5) is an envelope detecting circuit which picks out an envelope of the reproducing signal from an output of the preamplifier (3). An output signal of the envelope detecting circuit (5) is applied to a comparator (6) to be compared with a given constant level signal.
Reference numeral (7) is a memory control circuit which receives the output signal of the comparator (6) and generates a timing signal and an address signal for writing an output signal from the video signal processing circuit (4) on a field memory (8) in response to the output of the comparator (6). The video signal processing circuit (4) produces a synchronizing signal to the memory control circuit (7).
The field memory (8) is a dual-port memory (not shown in the drawing) having a random output and a serial output as an output port. Writing and reading operations can be performed asynchronously by using the serial port. In this case, a reading operation for reading a content of the field memory (8) is carried out asynchronously by the use of the signal port, while the reproducing signal from the video signal processing circuit (4) is written to the field memory (8).
Reference numeral (9) is a control head. A servo circuit (10) controls a capstan motor (11) and a reel motor (12) based on an output signal of the control head (9) to thereby control the running of the tape in each mode.
An operation of the video image reproducing apparatus constructed above will be described hereinbelow.
The video tape (1) is played back in a reverse direction at a high speed (of four times the recorded speed, for instance). FIGS. 2 and 3 are explanatory diagrams of the operation when the video tape (1) is played back in the reverse direction at four times the recorded speed. In FIG. 2, reference numeral (50) is a video track and reference characters (A) and (B) designate azimuth recording, respectively. The video head (2a) moves along the azimuth (A) whereas the video head (2b) moves along the azimuth (B).
When the video head (2a) traces along the azimuth shown by broken lines (d) in FIG. 2, the reproducing signal of the preamplifier (3) is as shown in FIG. (3a) due to the azimuth recording. However, the video head (2a) and the video head (2b) are used both in a first mode and a second mode in which a tape feeding speed is slower than that of the first mode. A head width is narrower than a tracking pitch in the first mode.
When the video head (2b) traces a locus indicated by broken lines (e) of FIG. 2, an output signal of the video head (2b) is as shown in FIG. 3(b). The contents of the two fields of FIG. 3(a) and (b) are interpolated with each other on a time base, so that one field image is obtained as shown in FIG. 3(c). The one field image is stored in the field memory (8) and displayed on a monitor. The envelope waveform shown in FIGS. 3(a) to 3(c) shows only an upper side of alternating waveforms which are symmetrical with respect to the upper and the lower sides.
Thus noiseless content of one field can be obtained in the case where there is no guard band in the video tape. The guard band is, however, formed on the video track (50) when the first mode recording is performed by the two above described system of two head type whose head width is narrower than the track pitch in the first mode. As a result, the video tracks (51) are created as shown in FIG. 4. When the video heads (2a) and (2b) trace on the video tracks (51) along loci (f) and (g) shown by broken lines of FIG. 4 respectively, the outputs of the preamplifier (3) are shown in FIGS. 5(a) and 5(b). The video signal of one field is as shown in FIG. 5(c) when the output signals of the preamplifier (3) are interpolated with each other on the time base.
It is, however, necessary to change a comparison potential of the comparator (6) when the video signals are interpolated on the time base as shown in FIGS. 3(c) and 5(c), respectively. Namely, the comparison potential must be set to a potential (S) shown in FIG. 3(c). This is because an image information at the time duration (J) is neglected when the comparing reference potential is set to a level (S1) higher than the level (S). This results in an occurrence of the noise bar in the time duration (J). On the contrary, the contents are rewritten in every field at the time duration (J), when the comparing voltage is set to a level (S2) lower than the level (S). Therefore, the image corresponding to the (J) portions are disturbed. Accordingly, the image information is reproduced smoothly by setting the comparing reference potential of the comparator (6) to the above described potential (S).
The noise bar, however, appears at the time durations (K) shown in FIG. 5(c) if the potential corresponding to the level (S) is set to the comparator (6) as in the case mentioned above. In this instance, the information cannot be obtained in the time duration (n) as a signal, even when the interpolated picture information is made continuous by employing, for example, zero (0) potential of the comparator (6). This results in degrading an S/N ratio and an occurrence of a noise having a certain noise width. In more detail, the envelope signal cannot be picked up sufficiently even when the potential of the comparator (6) is suitable, because the video heads (2a) and (2b) are also used for the second mode and their head widths are narrow. Therefore, the wrong S/N ratio portions appear in the (n) portions.
In view of the above, assuming that video heads whose head width is sufficiently large, are used, when the speed search is carried out in the second mode, it is disadvantageous that adjacent signals may be reproduced resulting in an occurrence of a noise in an image reproduced due to cross talk.